The present invention relates generally to the field of power control and, more particularly, to an apparatus and a method for limiting alternating current to a load by increasing the inductance of a control circuit.
The invention is particularly useful in the context of remote power controllers and remote-controlled circuit breakers which satisfy the requirements of military specifications MIL-P-81653B, dated Feb. 14, 1980, and MIL-C-83383B, dated Oct. 18, 1983. Controller devices of this type comprise output power switches which represent a low impedance between supply and load terminals in an "ON" state and a higher impedance in an "OFF" state. The state of the power switch normally conforms to that represented by an externally applied command signal. On detection of an electrical overload, the controller opens the circuit to isolate the load circuit from the supply or limits the fault current to a preselected maximum value.
Remote power controllers have application in the field of aerospace, in which it is necessary to prevent damage to aircraft wiring in the event of a load fault or a short circuit in the wiring system. Such controllers perform the combined functions of relays and circuit breakers in single and multiphase systems. They permit the ON/OFF control of electrical loads under normal conditions by signals from remote (pilot-operated) switches or a master control computer and, in the event of a load fault condition, open the circuit to the load. Both the magnitude and duration of the current in the load circuit are considered in determining whether an overcurrent condition exists. For example, a current equal to 600 percent of rated current might be tolerable for approximately 100 milliseconds, to account for the conditions encountered during motor startup, while a current of 140 percent of rated current might be acceptable for as long as several seconds during generator overvoltage surges.
Remote power controllers (RPC's) typically open and close power circuits by solid state switches operated in accordance with master control signals or trip signals based on the magnitude of an overcurrent condition and the time during which it is present. Remote controlled circuit breakers (RCCB's) are similar in design to RPC's but use electromechanical contacts instead of solid state switches. The switches and/or relays must be designed to withstand high short circuit currents before they open in order to prevent damage to the devices and to satisfy military specifications. Short circuit fault currents can be as high as 6000 amps (MIL-P-81653), requiring the devices to be greatly overdesigned for normal steady state conditions. For example, it is not unusual for a transistor or silicon-controlled rectifier (SCR) rated for 35 or 70 amps steady state to be used in an RPC for a circuit having a steady state current rating of only 3 amps.
Another disadvantage of some solid state switches, including SCR's, is that they switch between "ON" and "OFF" conditions only at a zero-current crossing. In a 400 hertz power system, zero-crossings occur every 1.25 milliseconds, whereas a short circuit current maximum can occur in a much shorter time. Thus, permanent damage can be done to a circuit before a conventional RPC acts.
Electromehanical relays of a conventional RCCB's are even slower to act in response to a fault (typically 10 to 30 milliseconds). They also deteriorate rapidly if called upon to open and close under high current conditions.
Therefore, it is desirable in many applications to provide an apparatus and a method for rapid limitation of fault currents.